• # question_answer A source is moving towards an observer with a speed of 20 m/s and having frequency of 240 Hz. The observer now moving towards the source with a speed of 20 m/s. Apparent frequency heard by the observer, is (velocity of sound = 340 m/s): A)  270 Hz          B)  540 Hz C)  135 Hz           D)  370 Hz

Here: Speed of the source ${{V}_{s}}=20m/s$ Frequency of sound $v=240\text{ }Hz$ Speed of the sound$v=340\text{ }m/s$ The apparent frequency heard by observer is given by $n=\left( \frac{\upsilon +{{\upsilon }_{0}}}{\upsilon -{{\upsilon }_{s}}} \right)n$ Or $n=\frac{340+20}{340-20}\times 240$ Or $n=\frac{360\times 240}{320}=270\,Hz$